One of the important areas of biomechanics studies the mechanical behavior of biological tissue and orthopedic elements to determine the etiology of diseases and the development of medical devices used for the treatment of different pathologies of the musculo-skeletal system (J. J. Garcia, D. H. Cortés. A nonlinear biphasic viscohyperelastic model for articular cartilage. Journal of Biomechanics 39, pages 2991-2998, 2006; W. Wilson, C. C. van Donkelaar, B. van Rietbergen, R. Huiskes. A fibril-reinforced poroviscoelastic swelling model for articular cartilage. Journal of Biomechanics 38, pages 1195-1204, 2005). Among these elements, we find external fixation devices, indispensable tools for treating bone fractures. There are several configurations of external fixation devices, among which we may cite the unilateral configuration, the ring configuration, and the combined configuration, which gives rise to the hybrid fixation device. Configuration versatility, stability, and rigidity are among the essential features of a good external fixation device. These permit adequate reduction of the fracture and the formation of a good osseous callus after installation (Georg N. Duda, Helmut Kirchner, Hans-Joachim Wilke, Lutz Claes. A method to determine the 3-D stiffness of fracture fixation devices and its application to predict inter-fragmentary movement. Journal of Biomechanics 31, pages 247-252, 1998). It is also desirable for the fixation device to not be too bulky or heavy.
One of the models used widely is the Sardi fixation device, comprising presses (elements connecting the screws that penetrate the bone fragments to one or more bars) that permit four degrees of freedom between nails and connecting bars; its double-bar configuration provides very good stability and rigidity to the ensemble but it is very difficult to install when seeking to adequately reduce the fracture, given that it only permits two degrees of freedom among the fragments. Moreover, there are the double-bar unilateral configurations, assembled with connecting presses with five degrees of freedom, like those generally offered commercially, which limit the possibility of correcting deformities, given that it is not possible to vary the distance between the axles of the connecting cylindrical elements. A poorly aligned fracture may cause complications in neighboring articulations due to degenerative articular disease, secondary to abnormal pressure on the articular cartilage (V L Caja, W Kim, S Larsson, EYS Chao. Comparison of the machanical performance of three types of external fixators: linear, circular and hybrid. Clinical Biomechanics. Vol. 10, Issue 8, Pages 401-406, 1995.).
The resistance and stability of the external fixation system are critical factors (Gardner T N, Simpson H, Kenwright J. Rapid application fracture fixators—an evaluation of mechanical performance. Clinical Biomechanics. 16(2):151-9, 2001). From this point of view, a double-bar system provides significantly greater stability than the single-bar system in which the slippage of a single connector implies the failure of the ensemble. Additionally, some studies have shown the influence of the rigidity of the fixation device on fracture healing (Gert D. Krischak, Andreas Janousek, Steffen Wolf, Peter Augat, Lothar Kinzl, Lutz E. Claes. Effects of one-plane and two-plane external fixation on sheep osteotomy healing and complications. Clinical Biomechanics 17, pages 470-476, 2002). For example, in experiments with animals it has been concluded axial inter-fragmentary displacements between 0.2-1 mm are optimal for fracture healing (Georg N. Duda, Helmut Kirchner, Hans-Joachim Wilke, Lutz Claes. A method to determine the 3-D stiffness of fracture fixation devices and its application to predict inter-fragmentary movement. Journal of Biomechanics 31, pages 247-252, 1998.).
In the patent literature, there are several devices for fracture fixation. For example, document WO2004/026103 describes a unilateral external fixation device that permits manipulation and fine adjustment of deformities by six degrees of freedom. The device is made up of an assembly bar and two moving joints, one on each side of the bar, each of which has two joints with a reduction gear mechanism with a concentric or axial helical mechanism and a locking mechanism to assure the device's degrees of freedom. The device is joined to the bone fragment through a press and one or more fastening screws.
Document WO1992/002184 presents an orthopedic manipulator for the reduction and stabilization of bone fractures through micrometric displacements by three degrees of freedom for both fragments and all the possible rotations via two sectors sliding one over the other reducing the opening angle; thus, permitting rotational and axial displacement of a portion of the bone.
Patent application WO1997/041790 describes an external fixation press adapted to fasten a locking screw to a connecting bar. The press includes a groove that transversely receives the connecting bar. A bolt is inserted through an opening transversely crossing said groove to join the connecting screw to the locking screw. Said connecting screw presents a surface attachment to the bar that fastens the connecting bar inside the groove increasing the press rigidity to prevent rotation around the bar and rotation of the connecting screw in the body of the press. Additionally, a device is described joining at least two connecting bars to guide the insertion of fastening screws between the bars. The device permits adjusting the distance between bars and presents different forms of union to the bars to facilitate installation of the press above or below the bars.
U.S. Pat. No. 6,428,540 introduces a device for repositioning fractured bone fragments comprising: a first press adapted to be joined to the bone by means of a first fastener; a second press adapted to be joined to the bone by means of a second fastener; three translational components associated to the presses that permit relative translational movement independent of the first press against the second press, along a first, second, and third translation axel where the three axels form an oblique-angle coordinated system; three rotation components associated to the presses permitting relative rotational movement independent of the first press against the second press, along a first, second, and third rotation axel; first and second parallel bars connecting the presses to the rotation and translation components, where the presses include a clamping jaw to receive the bone fasteners and a jaw that connects the clamping jaw to the parallel bars. The translation and rotation mechanisms permit altering the relative position of the presses in such a way that the device bears six degrees of freedom.
In spite of the different models existing commercially and in the literature, there is still a need for an external fixing device for bone trauma, which offers traumatologists greater versatility to adequately reduce the fracture; likewise, the configuration of the device must be adaptable during the different stages of surgery or during treatment through the incorporation of new elements or by eliminating others already assembled (screws, bars, presses, etc.).
The invention's external fixation device to stabilize bone introduces an adaptable configuration, which offers traumatology physicians greater versatility in placing nails and varying the distance between the axels of the cylindrical elements it connects—additional degree of freedom—, which permits adequately reducing the fracture, preserving the stability of the device through double-bar configurations. The versatility of the device permits modifying or adapting its configuration during the different stages of surgery or during treatment to allow, for example, including new elements (nails, bars, presses, etc.) or easily removing some already assembled elements without the need to completely disassemble the original configuration.
In the technical area, some of the advantages of the novel device from the present invention are: (1) It permits connecting a pair of screws from a bone fragment with a pair of screws from another bone fragment that are approximately on another plane, through a configuration of two bars and eight connecting presses. (2) Likewise, it permits connecting a pair of screws joined to the osseous diaphysis, located approximately on a plane, with another pair of screws from another fragment near the bone head, which are approximately on a transverse plane, perpendicular to the plane that approximately keeps the screws joined to the diaphysis, through a stable configuration of eight presses and three bars, without requiring other fastening elements. (3) Additionally, it permits connecting four or more osteosynthesis screws that are not perfectly aligned by using connecting presses and two bars. (4) It permits treating bone fractures to reduce the fracture without the need for the perfect alignment of the osteosynthesis screws joining the fastener to the bone fragments.